The impact of fish meal replacement with plant proteins on fish health is still poorly investigated. This study evaluates the effects of several diets containing graded levels of a mixture of gluten meal, soybean meal and soy protein concentrate on some immune parameters of turbot juveniles. The feeding trial was carried out at the aquaculture facilities of the Veterinary University of Bologna (Cesenatico, Italy). Turbot juveniles (9.7kept at 18in 500 l recirculating tanks (in triplicate) were fed four isonitrogenous (51%) and isolipidic diets (16%) (Skretting ARC, Stavanger, NO) for 9 weeks: a reference diet of 50% fish meal (FM50), and three diets containing 35%, 20% and 5% fish meal (FM35; FM20; FM5), with a proportional amount of vegetable ingredients. At the end of the trial, blood samples and spleen tissues were collected and analyzed for serum lysozyme concentration (lysoplate assay), differential leukocyte count (blood smears/May-Grunwald Giemsa) and quantification of splenic melano-macrophage centres (MMCs) (histology/image analyzer). One-way ANOVA and c2-test were applied respectively to humoral and cellular immune parameters to test the effects of dietary treatment. Plant protein diets induced significant changes in several immune parameters in turbot juveniles. Serum lysozyme was higher in turbot fed on FM35 and FM20 diets. The percentage of circulating phagocytes, mainly neutrophils, and splenic MMCs, increased, especially in fish fed FM5 and FM20 diets. No difference was observed in lymphocyte population. The increase in blood phagocytes could be responsible for the rise in serum lysozyme and interpretable as a stress or inflammatory response. Conversely, the lower lysozyme level measured in turbot fed FM5 diet in the face of a higher number of circulating phagocytes could be interpreted as a functional impairment of these cells. The increase in MMCs may be associated with a nutritional deficit, which is more severe in fish fed FM5. Long-term feeding trials are needed in order to gain further insight into the impact of dietary plant proteins on immune function and health in turbot.